The invention relates to surge arresters.
Electrical transmission and distribution equipment is subject to operating voltages within a fairly narrow range under normal conditions. However, system disturbances, such as lightning strikes, poor regulation, unbalanced loads, and switching surges, may produce momentary or extended voltage levels, especially on one or more phases of a multi-phase system, that greatly exceed the levels experienced by the equipment during normal operating conditions. These system variations often are referred to as overvoltage conditions and may lead to the damage of surge arresters installed to protect against transient overvoltages.
If not protected from transient current surges, critical and expensive equipment, such as transformers, switching devices, computer equipment, and electrical machinery, may be damaged or destroyed. Accordingly, system designers routinely use surge arresters to protect system components from dangerous overvoltage conditions.
A surge arrester is a protective device that is commonly connected in parallel with a comparatively expensive piece of electrical equipment so as to shunt or divert overvoltage-induced current surges safely around the equipment, thereby protecting the equipment and its internal circuitry from damage. The surge arrester normally operates in a high impedance mode that provides a relatively high impedance current path to ground. When exposed to a transient overvoltage condition, the surge arrester operates in a low impedance mode that provides a relatively low impedance current path to electrical ground (or earth). The impedance of the current path is substantially lower than the impedance of the equipment being protected by the surge arrester when the surge arrester is operating in the low impedance mode, and is otherwise substantially higher than the impedance of the protected equipment when in the high impedance mode.
Upon discharge of the transient overvoltage condition, the surge arrester returns to operation in the high impedance mode. This prevents normal current at the system frequency from following the surge current to ground (or earth) through the surge arrester.
Gapless surge arresters typically include an outer enclosure or housing made of an electrically insulating material, a pair of electrical terminals for connecting the arrester between a line-potential conductor and electrical ground (or earth), and an array of other electrical components that form a series electrical path between the terminals. These components typically include a series assembly of voltage-dependent, nonlinear resistive elements, referred to as varistors. A varistor is characterized by having a relatively high resistance when exposed to a normal operating voltage, and a much lower resistance when exposed to a higher voltage, such as is associated with a transient overvoltage condition. A metal-oxide varistor (xe2x80x9cMOVxe2x80x9d) is one type of varistor. In addition to varistors, a surge arrester may include one or more spark gap assemblies housed within or outside the insulating enclosure and electrically connected in series with the varistors.
In one general aspect, the invention features retrofitting a surge arrester having electrical connections to a source of power and to electrical ground with a series connected spark gap assembly to improve performance of the surge arrester. A spark gap module including at least one spark gap assembly sealed within a housing is provided. An electrical connection of the surge arrester is disconnected, and the spark gap module is connected between the electrical connection and the surge arrester.
Embodiments may include one or more of the following features. For example, the spark gap module may be connected between the surge arrester and the source of power, or between the surge arrester and electrical ground.
The surge arrester may be a gapless surge arrester. For example, the surge arrester may be a gapless distribution arrester having a 3-36 kV rating, and rated for normal duty (5 kA) or heavy duty (10 kA) operation, although not limited to these ratings.
The spark gap module may consist of one or more gap assemblies positioned between a pair of terminals, with the one or more gap assemblies and the terminals sealed within the housing. Each terminal may include a threaded bolt hole. The housing may be a porcelain or polymer housing that may or may not have defined weathersheds.
The spark gap assembly may include a resistive or capacitive graded gap structure. The gap structure may include electrodes separated by silicon carbide grading resistors, ceramic capacitors, or other impedance elements.
In another general aspect, the invention features a retrofit module for adding a spark gap assembly to a surge arrester to improve performance of the surge arrester. The module includes a housing, at least one spark gap assembly sealed within the housing, and structure for electrically connecting the spark gap assembly to a surge arrester, the structure being accessible from outside the housing.
Other features and advantages will be apparent from the following description, including the drawings and the claims.